1. Field of the Invention
The present invention relates generally to a method of diagnosing a cell proliferative disorder of breast tissue by determining the DNA methylation status of nucleic acids obtained a subject.
2. Background Information
Methylation has been shown by several lines of evidence to play a role in gene activity, cell differentiation, tumorogenesis, X-chromosome inactivation, genomic imprinting and other major biological processes (Razin, A., H., and Riggs, R. D. eds. in DNA Methylation Biochemistry and Biological Significance, Springer-Verlag, New York, 1984). In eukaryotic cells, methylation of cytosine residues that are immediately 5′ to a guanosine, occurs predominantly in cytosine-guanine (CG) poor regions (Bird, Nature, 321:209, 1986). In contrast, CpG islands remain unmethylated in normal cells, except during X-chromosome inactivation (Migeon, et al., supra) and parental specific imprinting (Li, et al., Nature, 366:362, 1993) where methylation of 5′ regulatory regions can lead to transcriptional repression. De novo methylation of the Rb gene has been demonstrated in a small fraction of retinoblastomas (Sakai, et al., Am. J. Hum. Genet., 48:880, 1991), and recently, a more detailed analysis of the VHL gene showed aberrant methylation in a subset of sporadic renal cell carcinomas (Herman, et al., Proc. Natl. Acad. Sci., U.S.A., 91:9700, 1994). Expression of a tumor suppressor gene can also be abolished by de novo DNA methylation of a normally unmethylated CpG island (Issa, et al., Nature Genet., 7:536, 1994; Herman, et al., supra; Merlo, et al., Nature Med., 1:686, 1995; Herman, et al., Cancer Res., 56:722, 1996; Graff, et al., Cancer Res., 55:5195, 1995; Herman, et al., Cancer Res., 55:4525, 1995).
Human cancer cells typically contain somatically altered nucleic acid, characterized by mutation, amplification, or deletion of critical genes. In addition, the nucleic acid from human cancer cells often displays somatic changes in DNA methylation (Fearon, et al., Cell, 61:759, 1990; Jones, et al., Cancer Res., 46:461, 1986; Holliday, Science, 238:163, 1987; De Bustros, et al., Proc. Natl. Acad. Sci., USA, 85:5693, 1988); Jones, et al., Adv. Cancer Res., 54:1, 1990; Baylin, et al., Cancer Cells, 3:383, 1991; Makos, et al., Proc. Natl. Acad. Sci., USA, 89:1929, 1992; Ohtani-Fujita, et al., Onco-gene, 8:1063, 1993). However, the precise role of abnormal DNA methylation in human tumorogenesis has not been established.
Aberrant methylation of normally unmethylated CpG islands has been described as a frequent event in immortalized and transformed cells, and has been associated with transcriptional inactivation of defined tumor suppressor genes in human cancers. This molecular defect has also been described in association with various cancers. CpG islands are short sequences rich in the CpG dinucleotide and can be found in the 5′ region of about half of all human genes. Methylation of cytosine within 5′ CGIs is associated with loss of gene expression and has been seen in physiological conditions such as X chromosome inactivation and genomic imprinting. Aberrant methylation of CpG islands has been detected in genetic diseases such as the fragile-X syndrome, in aging cells and in neoplasia. About half of the tumor suppressor genes which have been shown to be mutated in the germline of patients with familial cancer syndromes have also been shown to be aberrantly methylated in some proportion of sporadic cancers, including Rb, VHL, p16, hMLH1, and BRCA1 (reviewed in Baylin, et al., Adv. Cancer Res. 72:141-196 1998). Methylation of tumor suppressor genes in cancer is usually associated with (1) lack of gene transcription and (2) absence of coding region mutation. Thus CpG island methylation can serve as an alternative mechanism of gene inactivation in cancer.
Breast cancer is by far the most common form of cancer in women, and is the second leading cause of cancer death in humans. Despite many recent advances in diagnosing and treating breast cancer, the prevalence of this disease has been steadily rising at a rate of about 1% per year since 1940. Today, the likelihood that a woman living in North America will develop breast cancer during her lifetime is one in eight.
Breast cancer is often discovered at a stage that is advanced enough to severely limit therapeutic options and survival rates. Accordingly, more sensitive and reliable methods are needed to detect small (less than 2 cm diameter), early stage, in situ carcinomas of the breast. In addition to the problem of early detection, there remain serious problems in distinguishing between malignant and benign breast disease, in staging known breast cancers, and in differentiating between different types of breast cancers (e.g., estrogen dependent versus non-estrogen dependent tumors). Recent efforts to develop improved methods for breast cancer detection have focused on cancer markers such as proteins that are uniquely expressed (e.g., as a cell surface or secreted protein) by cancerous cells, or are expressed at measurably increased or decreased levels by cancerous cells compared to normal cells. Accordingly, the use of the methylation status of certain genes as a marker of cancer or cancerous conditions provides an additional weapon in early detection and prognosis of breast cancers.
Identification of the earliest genetic changes in cells associated with breast cancer is a major focus in molecular cancer research. Diagnostic approaches based on identification of these changes in specific genes are likely to allow implementation of early detection strategies and novel therapeutic approaches. Targeting these early changes might lead to more effective cancer treatment.